I agree that you are going to have a next to impossible time finding what you want for your budget. You can get digital point-and-shoots for around $150 in the 3-4 mp range, (they're harder to find though, most manufacturers are constantly increasing their resolution and keeping cameras at the same price, rather than lowering their prices, and there aren't too many cameras in this range that haven't been discontinued)
However, based on what you want to do, I don't think you'll be happy with a point-and-shoot at all. Because of the tiny size of the lens and sensor on these types of cameras, there is a limit on the image quality you can get out of them. Once you get past about 5 or 6 mp, you end up with so much noise reduction and other image enhancing going on in the camera to offset the limitations of teeny-tiny photosites, that your larger image is barely any different from the quality you'd get upsampling a smaller file from a lower resolution camera (provided that camera is from a good manufacturer and has decent image quality itself)
In addition to these image quality/ resolution concerns, the physical size of both the lens and sensor in a p-and-s also pretty much guarantees that you're never going to get the kind of separation between your foreground and background that you can get easily with an SLR.
Unfortunately again, DSLR's are much more expensive, the cheapest being probably in the $600-$1000 range.
If you are already an experienced photographer who just wants to go digital, one thing you could consider is buying a used body. These are still going to run you more than $150, but not as much more. If you already have lenses for a film SLR, they will most likely work on a digital body from the same manufacturer (one notable exception being pre-EF lenses from Canon) Many reputable photo supply stores like B and H Photo Video, Adorama, and many others sell used equipment with accurate assessments of it's wear level. You can also look for photo forums which often have a buy-and-sell board. Most new DSLR's on the market today are in the 8-10-12 mp range, which is great, but also not at all necessary to get good results. Beyond 4 or 5 mp, good exposure and composition are much more important than resolution. A skilled photographer with a 6 mp camera and a decent lens will outshine a rich amateur relying on his $8,000 16 mp Canon EOS 1Ds Mark II any day of the year.
Alternatively, you can buy a scanner with the ability to scan negatives (much much much better than scanning prints!) for around $150, and then take those into photoshop. The instant gratification of digital can be very nice, and also a good teaching aid, but when it comes right down to it the technique is the same, whicever medium you're using. If your budget is really tight, you may be better off waiting until you can afford the camera you want than being frustrated.
If all this is way over your head and you're just looking to start taking photos, why not buy an introductory Kodak or Samsung p-and-s with as much resolution as you can afford (I THINK both of those mfr's have cameras in the $150-$200 range) and just practice taking photos and using photoshop? you can have fun composing shots and getting instant feedback and meanwhile be saving up for a DSLR down the road. If you want to be able to learn more about lighting etc., the Canon A series cameras have a decent number of manual controls, and their G series have even more (though for a G-series you'll definitely have to go used to stay within your budget)
i know this is a really long winded reply but I hope it helps.
